This invention relates to forgings used for large land-based gas turbines, and particularly to large Alloy 718 forgings that are prone to a problem known as abnormal grain growth.
The forging of Alloy 718 involves heating a billet and forging it in one or many steps (also referred to as upsets) to the final required shape. The billet must be reheated before each upset. After forging, the shaped parts are solution treated at a high temperature (1700-1825F), and then aged at a lower temperature (1325-1150F) to develop strength. Under certain process conditions, Alloy 718 forgings develop abnormal grain growth when heated to the solution temperature. This has not been a serious problem for small forgings (as discussed below), but it has been a serious problem for large forgings which, for purposes of this invention, are those over 10,000 pounds in weight.
Abnormal grain growth, also referred to as secondary grain growth or critical grain growth, occurs when a few grains in the material grow to a very large size compared to neighboring grains. This occurrence alters the mechanical properties of the material. Specifically, not only does abnormal grain growth reduce fatigue resistance and yield strength of the material, it also impairs the ability to detect small defects by ultrasonic testing. Abnormal grain growth does however, improve creep resistance at high temperatures, and may therefore be desirable in certain instances.
The possibility of developing abnormal grain growth in Alloy 718 forgings has been known for some time. A prior document which describes abnormal grain growth in Alloy 718 and conditions which promote abnormal grain growth is the xe2x80x9cStudy of Secondary Grain Growth on 781 Alloyxe2x80x9d by J. F. Uginet and B. Pieraggi; The Minerals, Metals and Materials Society, 1997. Abnormal grain growth was not regarded as a serious problem in that process modifications were available that minimized or eliminated its occurrence. These process modifications, however, work well with small forgings but not with large forgings as defined above. More specifically, with small parts, one could:
1. Avoid low strains (amount of upsets in a forging step). There are some difficulties in doing this for large parts because the press capacity may allow only small upsets each time.
2. Use higher strain rates (related to the speed at which the top dies move). This again will not work for very large parts because the higher strain rates require higher press loads that would exceed the capacity of the largest presses in the world.
3. Avoid doing a solution treatment and do a direct age instead. This works out well for small parts because the cooling rate in air after completion of forging is adequate to ensure a fully solutioned structure for a small part. If air cooled after forging, the cooling rate at the center of very large parts will be very slow and will not have a fully solutioned structure. The absence of a fully solutioned structure means that the part will not develop high strength after the aging heat treatment. Therefore, after the solution treatment, the parts must be quenched in oil/water to retain a fully solutioned structure.
This invention involves the identification of a unique processing window for large Alloy 718 forgings which causes abnormal grain growth. By then avoiding this window, abnormal grain growth can be eliminated, thereby permitting large forgings that have a uniform grain structure. Alternatively, the process permits the formation of abnormal grain growth in selected areas when considered desirable.
Initially, a study on the effect of forging parameters was carried out using small specimens, but the process was made to simulate the processing of large forgings. It was observed that abnormal grain growth occurs under specific conditions of:
(a) starting grain size;
(b) forging temperatures;
(c) forging strains;
(d) forging strain rates;
(e) number of upsets; and
(f) solution treatment temperature.
In one embodiment of this invention, abnormal grain growth can be avoided by a forging process which takes into account the above factors, within the parameters disclosed herein.
In an alternative embodiment of the invention, where there is a need to intentionally create abnormal grain growth in any areas of a forging, items (c) and (f) are altered, as also described further herein.
More specifically, the present invention relates generally to a process for forging large components of Alloy 718 material comprising:
a) providing a billet with an average grain size between ASTM 0 and ASTM 3;
b) heating the billet to a temperature of between 1750xc2x0 F. and 1800xc2x0 F.;
c) upsetting the billet to obtain a component part with a minimum strain of 0.125 in at least selected areas of the part;
d) reheating the component part to a temperature between 1750xc2x0 F. and 1800xc2x0 F.;
e) upsetting the component part to a final configuration such that the selected areas receive no strains between 0.01 and 0.125;
f) solution treating the component part at a temperature of between 1725xc2x0 F. and 1750xc2x0 F.; and
g) aging the component part over predetermined times at different temperatures.
When there is a need to intentionally create abnormal grain growth in any areas of a forging, then steps e) and f) are changed only as follows.
e) finish forge the part to intentionally create strains of 0.01 to 0.125 in the required areas.
f) solution treat the component part at 1825-1850xc2x0 F.
The process in accordance with the invention has advantages over the prior art. Specifically, one can develop a control process which can eliminate abnormal grain growth and have a uniform grain structure specifically for large 718 alloy forgings. Alternatively, one can develop a control process which does produce abnormal grain growth intentionally in specific areas to meet specific property needs. This aspect can be used in both large and small forgings.